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Salad washing

There is no opportunity with fresh salads or fruit processing to carry out a thermal reduction step for control of microorganisms.

The Fresh Cut Fruit & Vegetables Industry (FFVI) is significantly different to Ready to Eat cooked foods; in that there is no opportunity with salads or fruit to carry out a thermal reduction step for control of microorganisms. Both pathogenic and non-pathogenic microorganisms have been shown to form intimate attachments and/or form biofilms on fruit and vegetable surfaces, as well as on food contact surfaces on processing equipment. Such interactions allow the microorganisms to effectively resist the antimicrobial activity of applied disinfectants and therefore provide a source of further contamination of produce.

The use of potable water instead of water containing chemical disinfection agents for washing fresh-cut vegetables is being advocated in some European countries. Washing with clean water has been shown to typically reduce microbial loading by 1 log.

This water if recirculated soon becomes heavily contaminated with microorganisms. The options are to use the fresh water once only and then send to drain; or to treat it with a disinfectant to control the level of microorganisms in the recirculating water. Although the disinfectant is primarily there to control microbial levels in the water it also seems to have an effect on the microbial levels on the washed product.

Chlorine based disinfectants

Chlorine-based disinfectants are most commonly used in the fresh cut fruit and vegetable industry. Benefits include their low cost, ease of use and numerous studies demonstrating the ability of chlorine to reduce microbial numbers by up to 2 logs.

The efficacy of chlorination on water disinfection and microbial load reduction on product surfaces may be enhanced using surfactants and by acidifying the solution to achieve a pH of 6.0 to 6.5; great care has to be taken to ensure this pH adjustment is controlled and the does not proceed to the release of toxic chlorine gas. Note: The dynamic balance of the two forms of hypochlorite in water changes dramatically between pH 6.5 and 8.0. The faster acting antimicrobial form, HOCl, exists as 80 to 95% of the “free chlorine” detected with the paper test strips at pH 6.5 to 7.0. This level drops to less than 20% at pH higher than 8.0.

Recently, further enhancement of disinfection has been achieved by using ultrasound equipment attached to wash tanks.

Nonetheless, there is a drive to minimise the use of chlorine in the industry due to environmental concerns, potential health implications of chlorine by-products and consumer reticence. Coupled with this is a need to reduce water consumption, whilst ensuring that the safety and quality of products is maintained.

Chlorine alternatives

Ozone has been proposed as a useful disinfectant solution in water. In principle, because it is a powerful oxidiser it should be effective as a disinfectant. However, it will be rapidly deactivated by any organic material it comes into contact with (for example food debris). Ozone leaves no residues to taint or contaminate food and passes only oxygen and water to the environment.

Peroxy acetic acid based disinfectant are effective against a wide spectrum of micro-organisms, including vegetative and sporing bacteria, moulds, spores, yeasts and viruses. They are suitable for a wide variety of vegetable, salad, and fruit washing operations, it is particularly suited to automated dosing systems. At typical use concentrations the biocidal efficacy of peroxy acetic acid based disinfectants (such as Crystal) is several orders of magnitude better than that of comparable concentrations of sodium hypochlorite. They are low foaming and can be used in soak and recirculation systems. Unlike hypochlorite based disinfectants, they do not cause pitting corrosion, and do not precipitate brown stains of iron and manganese, and are effective without the need to modify pH by the addition of organic acids. Peroxy acetic acid based disinfectants break down to form water and acetic acid; no chlorinated organic compounds are produced.